Solid-state imaging device and method for manufacturing the same

ABSTRACT

A solid-state imaging device having a high sensitivity and a structure in which a miniaturized pixel is obtained, and a method for manufacturing the solid-state imaging device in which an interface is stable, a spectroscopic characteristic is excellent and which can be manufactured with a high yield ratio are provided. 
     The solid-state imaging device includes at least a silicon layer formed with a photo sensor portion and a wiring layer formed on the front-surface side of the silicon layer, and in which light L is made to enter from the rear-surface side opposite to the front-surface side of the silicon layer and the thickness of the silicon layer  4  is 10 μm or less. Also, the method for manufacturing the solid-state imaging device at least includes the steps of: forming a semiconductor region of a photo sensor portion in a silicon layer of a layered substrate in which a silicon substrate, an intermediate layer and a silicon layer are laminated; bonding a first supporting substrate onto the silicon layer; removing the silicon substrate and the intermediate layer; forming thereafter a wiring portion above the silicon layer; bonding a second supporting substrate onto the wiring portion, and removing the first supporting substrate to make the silicon layer exposed.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.10/978,754, filed Nov. 1, 2004, the entirety of which is incorporatedherein by reference to the extent permitted by law. The presentinvention claims priority to Japanese patent application No. 2003-374627filed in the Japanese Patent Office on Nov. 4, 2003, the entirety ofwhich also is incorporated by reference herein to the extent permittedby law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state imaging device and amethod for manufacturing the same, and particularly relates to what iscalled a solid-state imaging device of a back-illuminated type and amethod for manufacturing the same.

2. Description of the Related Art

With a high density integration of a semiconductor device, a transistorand other semiconductor elements have been tried to be more miniaturizedand to raise mounting density more than before.

Therefore, in a CMOS image sensor (CMOS type solid-state imagingdevice), it is required that a pixel is made minute and an element ismade to be highly integrated.

However, in a conventional CMOS image sensor in which light detection isperformed by irradiating a photo sensor portion with light from a lensformed on a wiring portion through the space between wiring layers, thephoto sensor portion can not be irradiated with an enough amount oflight, because an eclipse of incident light due to the obstacle such asa wiring layer occurs and an aperture ratio of the photo sensor portionis made small in accordance with the high density integration of deviceto make the pixel become minute. Accordingly, such problems as lowersensitivity and more shading may occur.

Therefore, the photo sensor portion is irradiated with light from therear side (opposite side to the wiring portion), so that an effective100% ratio of aperture can be achieved and sensitivity can besufficiently raised without an influence of an obstacle such as a wiringlayer.

Accordingly, a CMOS image sensor in which the photo sensor portion isirradiated with light from the rear-surface side (opposite side to thewiring portion), what is called a back-illuminated type CMOS imagesensor, has been developed.

Then, in the back-illuminated type CMOS image sensor, it is consideredthat a silicon layer of the photo sensor portion is made thin to obtainhigher sensitivity (refer to, for example, patent document 1 or patentdocument 2).

FIG. 1 is a vertically sectional view schematically showing aback-illuminated type CMOS image sensor to which the above describedconstruction is applied.

In a MOS sensor 100, since a silicon substrate 101 including photosensor portions 102 is made thin, sensitivity to incident light L can beraised.

Further, since the incident light L is not blocked even if wiring layers104 are formed on the photo sensor portions 102, flexibility in layoutof the wiring layers 104 can be obtained. Accordingly, the high densityintegration of an element can be obtained by forming the wiring layer104 to be multi-layered and by minimizing an area of a pixel.

A method for manufacturing the back-illuminated type image sensor havingsuch a thin silicon layer is considered, in which, for example, after aphoto diode of a photo sensor portion is formed in a silicon substrate,the silicon substrate is ground from the back thereof to be thin.

Patent document 1: Japanese Published Patent Application No. H6-77461(FIG. 3)

Patent document 2: Japanese Published Patent Application No. H6-283702(FIG. 2)

However, in the above-described manufacturing method, an interface ofthe silicon substrate after grinding the back surface becomeselectrically unstable. Also, a mechanical damage may affect the siliconsubstrate.

As a result, dark current is caused due to the above problems.

Hence, an image sensor of the back-illuminated type having theabove-described structure has a limitation in use such as requiring acooling process.

Furthermore, light absorption depends upon the thickness of a siliconlayer; however, since the thickness is made thin by grinding, controlfor the thickness of the silicon layer (silicon substrate) of the photosensor portion becomes deteriorated, so that the spectroscopiccharacteristic as the sensor tends to disperse.

As a result, the manufacturing yield ratio becomes deteriorated, and thecost may increase because of such a problem.

Therefore, the back-illuminated type structure has been used only forlimited purposes, though the sensitivity of the photo sensor portioncould be improved.

In addition, a conventional image sensor of the back-illuminated typestructure has several ten micro-meter of the thickness of a siliconlayer in which the photo sensor portion performing the photo-electricconversion is formed; and when a pixel is miniaturized, theelectric-charge of a signal diffuses between adjacent pixels to generatemixed color.

Accordingly, it is difficult to realize a minute pixel.

To solve the above-described problems, the present invention provides asolid-state imaging device having a structure in which sensitivity ishigh, almost no shading occurs and a pixel can be miniaturized.

Further, the present invention provides a method for manufacturing asolid-state imaging device, in which the solid-state imaging devicehaving a stable interface and an excellent spectroscopic characteristiccan be manufactured with a high yield ratio.

SUMMARY OF THE INVENTION

A solid-state imaging device according to the present invention includesat least a silicon layer having a photo sensor portion in whichphoto-electric conversion is performed and a wiring layer formed on thefront-surface side of the silicon layer, in which light is made to enterfrom the rear-surface side that is opposite to the front-surface side ofthe silicon layer and the thickness of the silicon layer is 10 μm orless.

According to the above-described present invention, a structure of whatis called a back-illuminated type is formed with the structure includingat least a silicon layer formed with a photo sensor portion and a wiringlayer formed on the front-surface side of the silicon layer, in whichlight is made to enter from the rear-surface side of the silicon layer.Further, since the thickness of the silicon layer is 10 μm or less, highsensitivity is obtained in the wide range of wavelength includinginfrared range and a drift electric-field of approximately 200 mV/μm ormore can be formed with a design having the range of drive voltage (2.5Vto 3.3V) that is conventionally used.

Furthermore, since the thickness of the silicon layer is thinner thanthe conventional ones, the distance between a lens and a semiconductorregion of the photo sensor portion can be shortened.

A solid-state imaging device according to the present invention includesat least a silicon layer having a photo sensor portion in which thephoto-electric conversion is performed and a wiring layer formed on thefront-surface side of the silicon layer, in which light is made to enterfrom the rear-surface side that is opposite to the front-surface side ofthe silicon layer and the thickness of the silicon layer is 5 μm orless.

According to the above-described present invention, a structure of whatis called a back-illuminated type is formed with the structure includingat least a silicon layer formed with a photo sensor portion and a wiringlayer formed on the front-surface side of the silicon layer, in whichlight is made to enter from the rear-surface side of the silicon layer.Further, since the thickness of the silicon layer is 5 μm or less, highsensitivity is obtained in the visible light range and a driftelectric-field with an approximate intensity of 400 mV/μm or more can beformed with a design having the range of drive voltage (2.5V to 3.3V)that is conventionally used.

Furthermore, since the thickness of the silicon layer is further thinnerthan the conventional ones, the distance between a lens and asemiconductor region of the photo sensor portion can further beshortened.

A method for manufacturing a solid-state imaging device according to thepresent invention is the method in which a layered substrate formed witha silicon substrate, an intermediate layer and a silicon layer laminatedis used, including at least the steps of: forming a semiconductor regionof a photo sensor portion in the silicon layer of the layered substrate;bonding a first supporting substrate to the silicon layer; removing thesilicon substrate and the intermediate layer of the layered substrate;then forming above the silicon layer a wiring portion having a wiringlayer in an insulative layer; bonding a second supporting substrate tothe wiring portion; and removing the first supporting substrate to makethe silicon layer exposed.

According to the above-described method for manufacturing a solid-stateimaging device of the present invention, since the wiring portion isformed on the silicon layer and the first supporting substrate isremoved to make the silicon layer exposed, the rear-surface side of thesilicon layer, which is opposite to the front-surface side in which awiring portion is formed, is exposed to obtain the back-illuminated typestructure in which light enters from the rear-surface side.

Further, since a layered substrate in which a silicon substrate, anintermediate layer and a silicon layer are laminated is used and asemiconductor region of a photo sensor portion is formed in the siliconlayer of the layered substrate, an interface of a silicon layer on whichthe semiconductor region of a photo sensor portion is formed becomescomparatively stable, so that the spectroscopic characteristic of asolid-state imaging device can easily be stabilized by controlling thethickness of a silicon layer.

Moreover, with respect a silicon layer where a semiconductor region isformed, since a wiring portion is formed on the front-surface sidethereof and the first supporting substrate is bonded to the rear-surfaceside and then the first supporting substrate is removed, there is noneed to grind the silicon layer and a mechanical damage can be preventedfrom affecting the silicon layer.

Further, upon use of a layered substrate including a thin silicon layer,a solid-state imaging device having the above-described solid-stateimaging device of the present invention, that is, having a structure ofa solid-state imaging device in which the silicon layer including asemiconductor region of a photo sensor portion is thin to be 10 μm orless (or 5 μm or less) can be manufactured.

A method for manufacturing a solid-state imaging device according to thepresent invention is the method in which a layered substrate formed witha silicon substrate, an intermediate layer and a silicon layer laminatedis used, including at least the steps of: forming a semiconductor regionof a photo sensor portion in the silicon layer of the layered substrate;forming on the silicon layer a wiring portion having a wiring layer inan insulative layer; then bonding a supporting substrate to the wiringportion; and removing the silicon substrate and the intermediate layerto make the silicon layer exposed.

According to the above-described method for manufacturing a solid-stateimaging device of the present invention, since the wiring portion isformed on the silicon layer and the silicon substrate and theintermediate layer of the laminated substrate are removed to make thesilicon layer exposed, the rear-surface side of the silicon layer, whichis opposite to the front-surface side in which a wiring portion isformed, is exposed to obtain the back-illuminated type structure inwhich light enters from the rear-surface side.

Further, since a layered substrate in which a silicon substrate, anintermediate layer and a silicon layer are laminated is used and asemiconductor region of a photo sensor portion is formed in the siliconlayer of the layered substrate, an interface of a silicon layer on whicha semiconductor region of a photo sensor portion is formed becomescomparatively stable, so that the spectroscopic characteristic of asolid-state imaging device can easily be stabilized by controlling thethickness of a silicon layer.

Moreover, with respect to a silicon layer where a semiconductor regionis formed, since a wiring portion is formed on the front-surface sidethereof and the silicon substrate and the intermediate layer areremoved, there is no need to grind the silicon layer and a mechanicaldamage can be prevented from affecting the silicon layer.

Further, upon use of a layered substrate including a thin silicon layer,a solid-state imaging device having the above-described solid-stateimaging device of the present invention, that is, having a structure ofa solid-state imaging device in which the silicon layer including asemiconductor region of a photo sensor portion is thin to be 10 μm orless (or 5 μm or less) can be manufactured.

According to a solid-state imaging device of the present invention,since what is called a back-illuminated type structure is formed,sensitivity can be improved and the occurrence of shading in thesurrounding pixels can be controlled; and further, since the thicknessof the silicon layer is 10 μm or less, a high sensitivity is obtained inthe wide range of the wavelength including infrared regions, and asufficient drift electric-field is formed to make the read-out of theelectric-charge to the front-surface side carried out securely.

Further, since the distance between a lens and a semiconductor region ofthe photo sensor portion can be shortened, the occurrence of mixed colorcaused by light incident on adjacent pixels can be restrained.

According to a solid-state imaging device of the present invention,since what is called a back-illuminated type structure is formed,sensitivity can be improved and the occurrence of shading in thesurrounding pixels can be restrained; and further, since the thicknessof the silicon layer is 5 μm or less, a high sensitivity can be obtainedin the range of visible light, and the drift electric-field with enoughintensity can be formed to make the electric-charge read-out to thefront-surface side.

Further, since the distance between a lens and a semiconductor region ofthe photo sensor portion can be shortened, the occurrence of mixed colorcaused by light incident on adjacent pixels can be controlled.

Furthermore, an ion implantation when forming a semiconductor region ofa photo sensor portion can easily be performed using, for example, onlya photo resist as a mask.

Therefore, according to the present invention, a solid-state imagingdevice of the back-illuminated type structure in which a highsensitivity is obtained and the electric-charge can be read out to thefront-surface side can be realized.

Further, the flexibility in the layout of a wiring layer and design canbe obtained due to the back-illuminated type structure, and theoccurrence of mixed color caused by light incident on adjacent pixelscan be restrained due to a thin silicon layer, so that a pixel caneasily be miniaturized. With the miniaturization of pixels of thesolid-state imaging device, the solid-state imaging device can be highlyintegrated and be small-sized.

Moreover, in each of the above-described respective solid-state imagingdevices of the present invention, when an element isolation region isformed between each pixel of said photo sensor portion in the wholethickness direction, the pixels can be electrically separated by theelement isolation region and color electrically mixed with adjacentpixels can be prevented.

Further, in each of the above-described solid-state imaging devices ofthe present invention, when a region of a second conduction type isformed in the vicinity of an interface of the silicon layer on therear-surface side, which is opposite to a region of a first conductiontype constituting a photo sensor portion, a dark current generated inthe vicinity of the interface of the silicon layer on the rear-surfaceside can be reduced.

According to the method for manufacturing a solid-state imaging deviceof the present invention, since the first supporting substrate is bondedonto a silicon layer; a wiring portion is formed above the silicon layerafter a silicon substrate and an intermediate layer of a layeredsubstrate are removed; the second supporting substrate is bonded ontothe wiring portion; and the first supporting substrate is removed tomake the silicon layer exposed, the interface of the silicon layerbecomes stable and the spectroscopic characteristic of a solid-stateimaging device is stabilized by controlling the thickness of the siliconlayer, so that a solid-state imaging device with an excellentspectroscopic characteristic can be manufactured with a high yieldratio.

Further, upon use of a layered substrate having a thin silicon layer, asolid-state imaging device having a thin silicon layer in which asemiconductor region of a photo sensor portion is formed can easily bemanufactured.

Particularly, when the first supporting substrate is bonded to a siliconlayer, the heat treatment at a comparatively high temperature such as atabout 1100 degree-centigrade is executed to make an impurity of asemiconductor region of the silicon layer activated and to make thecrystallinity on the interface of the silicon layer improved, so that asolid-state imaging device scarcely having noise can be manufactured.

According to the method for manufacturing a solid-state imaging deviceof the present invention, since a wiring portion is formed above asilicon layer; a supporting substrate is bonded onto the silicon layer;and a silicon substrate and an intermediate layer of a layered substrateare removed to make the silicon layer exposed, the interface of thesilicon layer becomes stable and the spectroscopic characteristic of asolid-state imaging device can be stabilized by controlling thethickness of the silicon layer, so that a solid-state imaging devicewith an excellent spectroscopic characteristic can be manufactured witha high yield ratio.

Further, when a layered substrate having a thin silicon layer is used, asolid-state imaging device having a thin silicon layer in which asemiconductor region of a photo sensor portion is formed can easily bemanufactured.

Particularly, since the heat treatment at a comparatively lowtemperature can be performed for manufacturing, influences of the heattreatment to a transistor and others can be reduced, so that thetransistor with a narrow pitch is formed to easily provide a minutepixel. Also, increase in the number of processes can be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic and vertically sectional view of aback-illuminated type CMOS image sensor;

FIG. 2 is a schematic constitutional view (showing a vertical section)of a solid-state imaging device according to an embodiment of thepresent invention;

FIG. 3 is a vertically sectional view of a relevant portion of thesolid-state imaging device of FIG. 2;

FIG. 4 is a characteristic curve showing the relationship between thethickness of a silicon layer and the dependence of the quantumefficiency on the wavelength of incident light in a photo sensorportion;

FIG. 5 is a characteristic curve showing the relationship between thethickness of a silicon layer and the dependence of the quantumefficiency on the wavelength of incident light in the photo sensorportion;

FIG. 6 is a view showing the spectroscopic characteristic of an infraredray cut-off filter;

FIG. 7 is a vertically sectional view of an SOI substrate;

FIGS. 8A to 8C are manufacturing process views showing an embodiment ofa method for manufacturing a solid-state imaging device according to thepresent invention;

FIGS. 9A to 9C are manufacturing process views showing an embodiment ofa method for manufacturing a solid-state imaging device according to thepresent invention;

FIGS. 10A and 10B are manufacturing process views showing an embodimentof a method for manufacturing a solid-state imaging device according tothe present invention;

FIGS. 11A and 11B are manufacturing process views showing an embodimentof a method for manufacturing a solid-state imaging device according tothe present invention;

FIGS. 12A to 12C are manufacturing process views showing anotherembodiment of a method for manufacturing a solid-state imaging deviceaccording to the present invention; and

FIGS. 13A to 13C are manufacturing process views showing anotherembodiment of a method for manufacturing a solid-state imaging deviceaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a schematic constitutional view (showing a verticalsection) of a solid-state imaging device of an embodiment of the presentinvention. In this embodiment, the present invention is applied to aCMOS image sensor (a CMOS type solid-state imaging device).

A solid-state imaging device 1 includes a supporting substrate 2, awiring portion 3, a silicon substrate 4, a color filter 5 and an on-chiplens 6, which are formed from the front-surface side in this order.

In the wiring portion 3, a plurality of wiring layers 12 are formed withan insulative layer 11 formed therebetween. A thin insulative coating 13which functions as a gate insulative film is formed between the wiringportion 3 and the silicon substrate 4, and a gate electrode 14 is formedon the front-surface side of the insulative layer 13 for the read-out ofthe electric-charge.

An N-type region 17 constituting a photo diode of a photo sensor portionis formed in the silicon substrate thickly in the thickness direction,and a positive electric charge storage region (P⁺ region) 16 is formedat a position on the front-surface side of the N-type region 17. Also,an N-type floating diffusion (FD) 15 is formed under the gate electrode14 with the read-out region in between.

Though not shown in the figure, the supporting substrate 2 and thewiring portion 3 are bonded by an adhesive layer or others. A siliconsubstrate for example can be used as the supporting substrate 2. Othermaterials can be used for the material of the substrate as long ashaving a favorable planarity and having a small difference in thermalexpansion rate from that of silicon.

Then, light L is made to enter from the side of the lens 6, namely therear-surface side opposite to the side of the wiring portion 3, toprovide a CMOS image sensor of what is called a back-illuminated type.

A read-out transistor is composed of the gate electrode 14, an end ofthe N-type region 17 and the floating diffusion 15.

Further, other transistors in a pixel and peripheral circuit elementsare formed on the front-side of the silicon substrate 4 in othersections not shown.

Particularly, in this embodiment, the thickness D of the silicon layer(silicon substrate) 4 where the photo censor portion is formed is set to10 μm or less. Preferably, the thickness D of the silicon layer 4 is setto 5 μm or less.

With the above structure, since the thickness D of the silicon layer 4is thinly formed, the occurrence of mixed color caused by light incidenton the adjacent pixels can be reduced and also a high sensitivity can beobtained.

Further, since a drift electric-field of approximately 200 mV/μm or morecan be formed when designed in the range of the drive voltage (2.5V to3.3V) conventionally used in a CMOS image sensor, the read-out of theelectric-charge to the front-surface side is securely performed withthis electric-field.

In addition, noise caused by the irradiation of light is equal to orless than that of a CMOS type solid-state imaging device of afront-illuminated type structure.

A high sensitivity can be obtained in the wide range of wavelengthincluding infrared-ray region, when the thickness D of the silicon layer4 is set to 10 μm or less.

A high sensitivity can be obtained in the range of visible light, whenthe thickness D of the silicon layer 4 was set to 5 μm or less.

Further, when designed having the range of the above-described drivevoltage, a drift electric-field of approximately 400 mV/μm or more canbe formed, so that the read-out of the electric-charge to thefront-surface side is carried out easily.

When the thickness D of the silicon layer 4 is set to 5 μm or less, anadvantage result that the manufacturing becomes easy can be obtained.

When the thickness D of the silicon layer 4 exceeds 5 μm, it isnecessary to execute an ion implantation with a super high energy and toform a hard musk of the oxide coating for forming the N-type region 17in the structure shown in FIG. 2.

On the contrary, when the thickness D of the silicon layer 4 was set to5 μm or less, manufacturing can be carried out easily, because the ionimplantation for forming the N-type region 17 can be performed using aresist mask.

Further, in the solid-state imaging device 1 according to the embodimentof the present invention, a P⁺ region (a highly concentrated p-typeregion) 18 is formed as an element isolation region at the positionbetween the N-type regions 17 of the photo sensor portion of theadjacent pixels in the whole depth direction.

Therefore, the N-type region 17 of each pixel can be electricallyseparated and an electrically mixed color between the adjacent pixelscan be prevented.

Furthermore, in the solid-state imaging device 1 according to theembodiment of the present invention, the P⁺ region 19 is also formed onthe rear-surface side of the N-type region 17, in other words, on theside of the color filer 5.

Accordingly, a dark current caused by the interfacial level on therear-surface side of the silicon layer 4 can be reduced.

In the solid-state imaging device 1, as shown in a vertical sectionaround the photo sensor portion of FIG. 3, the incident light isconverted into the electric-charge at a position comparatively deep inthe N-type region of the photo sensor portion (at the portion on therear-surface side), and the electric-charge e⁻ moves to thefront-surface side as shown by an arrow in FIG. 3. This movement isperformed smoother as much as the above-described drift electric-fieldhas a large area.

Then, when the gate electrode 14 becomes the ON state, theelectric-charge e⁻ is read out to the floating diffusions 15.

Here, in the solid-state imaging device 1 of the structure shown in FIG.2, the relationship between the thickness D of the silicon layer and thedependence of the quantum efficiency on the wavelength of the incidentlight L in the photo sensor portion is measured.

The relationship between the thickness D (μm) of the silicon layer andthe dependence of the quantum efficiency on the wavelength (nm) of theincident light L in the photo sensor portion is shown in FIGS. 4 and 5.FIG. 4 shows the thickness D of the silicon layer and the quantumefficiency of the whole silicon layer of that thickness. FIG. 5 showsthe quantum efficiency (an absorption ratio in each portion) in thethickness range of every 1 μm measured from the side of incident light,and for example the thickness between 2 μm and 3 μm is plotted at 2.5μm.

According to FIG. 4, blue (the wavelength of around 400 nm) is absorbedby 100% at 2 μm or less and green (the wavelength of around 550 nm) isabsorbed by 100% at approximately 5 μm, respectively. Red (thewavelength of around 750 nm) is not absorbed by 100% even at 10 μm.

According to FIG. 5, the absorption ratio of red (750 nm in wavelength)is 2% at the maximum in the depth of 4.75 μm to 5.25 μm (the diffusionlayer of a transistor is assumed). Green and blue are negligibly small.

Also, in the solid-state imaging device for the picture applicationwhich is seen with human eyes, an infrared ray cut-off filter isprovided to prevent the infrared ray from entering.

FIG. 6 shows the spectroscopic characteristic of the infrared raycut-off filter. In FIG. 6, both the infrared ray cut-off filter of thedeposition type and the infrared ray cut-off filter of the absorptiontype are shown. The deposition type is conventionally used, and light ofthe longer wavelength than 650 nm has a cut-off characteristic, thoughlight of the wavelength 650 nm or less is almost transmitted, as shownin FIG. 6.

Therefore, in the solid-state imaging device for the picture applicationseen with human eyes, the sensitivity on the side of the wavelengthlonger than 650 nm is not required. Note that, in the case where theabove solid-state imaging device is used for monitoring, it is desirableto have the sensitivity to infrared range.

Even if the thickness D of the silicone layer 4 is 5 μm or less, thesensitivity to light of the wavelength 650 nm or less can be obtainedsatisfactorily, so that the sufficiently high sensitivity is obtained inthe solid-state imaging device for the picture application seen by humaneyes, when the structure of this embodiment of the present invention isemployed.

Further, the occurrence of mixed color by the diffractive light in theCMOS image sensor of the front-illuminated type structure was measuredby analyzing the two-dimensional wave, and was found that severalpercentages thereof existed.

On the other hand, almost no mixed color by the diffractive light hasoccurred in the CMOS image sensor of the back-illuminated typestructure, and the occurrence thereof was less than the limit detectedby analyzing the wave (0.1% or less).

However, in the CMOS image sensor of the back-illuminated typestructure, light entered from the rear-surface side may affect anelement (for example, transistor) on the front surface, and may causenoise.

Hence, with the silicon layer of, for example, 5 μm in thickness, aninfluence upon an element (a transistor for, example) on thefront-surface side caused by the incident light from the rear surfacecan be restrained, and a total amount of noise can be reduced less thanthat of CMOS image sensor of the front-illuminated type structure.

According to the structure of the solid-state imaging device 1 of theabove-described embodiment of the present invention, the on-chip lens 6and others are disposed on the side (rear-surface side) opposite to theside (front-surface side) of the wiring portion 3 of the silicon layer 4where the photo sensor portion is formed, and the back-illuminated typestructure in which the light L is made to enter from the rear-surfaceside is employed, so that there is no wiring layer 12 between theon-chip lens 6 and the photo sensor portion, thus the loss of theincident light by the wiring layer 12 does not occur. Accordingly, theamount of incident light can be increased without changing the area ofthe photo sensor portion; and also, it becomes possible to increase thearea of the photo sensor portion and to set the shape of a pattern ofthe N-type region 17 to make light easily enter, so that the sensitivitycan be improved. Further, the occurrence of shading in the adjacentpixels can be restrained.

Due to the back-illuminated type structure, in this solid-state imagingdevice shown in FIG. 2, there is no need to pass light through thewiring portion, so that the degree of flexibility in design and thelayout of the wiring layer 12 may increase and thus, for example, thecoating thickness of the wiring layer 13 and the resistance can beoptimized.

Accordingly, the solid-state imaging device 1 of each pixel isminiaturized to attain the high density integration and theminiaturization. With the front-illuminated type structure, it isdifficult to make a CMOS image sensor including one million pixels ormore; however, with the structure of this embodiment, a CMOS imagesensor including one million pixels or more can easily be obtained.

Further, according to the structure of the solid-state imaging device 1of this embodiment, since the thickness D of the silicon layer 4 is 10μm or less, or preferably, 5 μm or less, the thickness D of the siliconlayer 4 becomes even thinner compared with the conventionalback-illuminated type structure which has the silicon layer ofapproximately several ten μm in thickness, and thus, the distancebetween the lens 6 and the N-type region 17 of the photo sensor portioncan be shortened more, so that the sensitivity can be improved as aresult, and also the occurrence of mixed color caused by incident lighton the adjacent pixels can be restrained, even if miniaturization of apixel is performed.

Then, with the thickness of silicon layer 4 reduced, the driftelectric-field can be formed with further intensity when the solid-stateimaging device is designed in the range of a conventional voltage (from2.5V to 3.3V), and the electric-charge to which the photo-electricconversion was performed at the rear-surface side can easily be read outto the front-surface side.

Accordingly, even if the amount of the electric-charge accumulated inthe photo sensor portion increases, the read-out of the electric-chargecan be performed satisfactorily, so that the accumulated electric-chargeincreases and the dynamic range is improved.

Furthermore, according to the structure of the solid-state imagingdevice 1 of this embodiment, the P⁺ region 18 is formed as the elementisolation region in the whole thickness direction of the silicon layer 4at the position between the N-type regions 17 constituting the photodiode of the photo sensor portion of each pixel, so that the pixels canbe separated electrically and electrically mixed color with the adjacentpixels can be prevented.

Furthermore, according to the structure of the solid-state imagingdevice 1 of this embodiment, the p⁺ region 19 is also provided on therear-surface side of the N-type region 17 of the silicon layer 4, sothat what is called a HAD (Hole Accumulated Diode) structure is formedon the rear-surface side as well, similarly to the front-surface side(the positive charge accumulating region 16).

Therefore, the occurrence of dark current generated in the vicinity ofthe interface of the silicon layer 4 on the rear-surface side can berestrained.

Further, the solid-state imaging device 1 of this embodiment is the CMOSimage sensor (CMOS-type solid-state imaging device), so that theoccurrence of smear which becomes a problem in the CCD solid-stateimaging device does not occur.

Next, as an embodiment of the method for manufacturing the solid-stateimaging device of the present invention, a method for manufacturing thesolid-state imaging device having the back-illuminated type structuresimilar to the solid-state imaging device 1 of FIG. 2 is explained.

In this embodiment, as shown in the vertical section in FIG. 7, an SOIsubstrate 24 is used, in which on a silicon substrate 23 a silicon layer21 is formed with a silicon oxide coating (SiO₂ coating) as anintermediate layer 22 in between.

With respect to the SOI substrate 24, the whole thickness thereof is 725μm or less, for example, and the thickness of the silicon layer 21 is 10μm or less (preferably 5 μm or less).

First, as shown in FIG. 8A, an N-type region 25 which becomes a mainportion (a portion of the rear-surface side) of the N-type region 17constituting the photo diode, and a P⁺ region 19 on the rear-surfaceside are respectively formed in the silicon layer 21 of the SIOsubstrate 24 by ion implantation. In addition, a matching mark 26 usedfor positioning a color filter and an on-chip lens is formed together.

Next, as shown in FIG. 8B, an adhesive layer 32 is formed on a surfaceof a first supporting substrate 31, and the first supporting substrate31 is adhered to the silicon layer 21 of the SOI substrate 24 with theadhesive layer 32 in between. Then, the heat treatment at, for example,a temperature of 1100 degree centigrade is performed for bonding. Atthis time, impurities in the N-type region 25 and in the P⁺ region 19 ofthe silicon layer 21 are activated.

Next, as shown in FIG. 8C, a wafer is positioned upside down.

Subsequently, the silicon substrate 23 and the intermediate layer 22that are on the silicon layer 21 are sequentially removed using aback-grind method, a CMP (Chemical Mechanical Polishing) method, awet-etching method or the like, for example. As a result, the siliconlayer 21 is exposed as shown in FIG. 9A.

Next, a gate electrode 14 of a read-out transistor is formed on thesilicon layer 21 with a thin insulative layer in between. Further, withrespect to the silicon layer 21, ion implantation of an N-type impurityis performed from the front-surface side to form an N-type region 27which becomes the front-surface side portion of the rest of the N-typeregion 17 constituting the photo diode, and a floating diffusion 15composed of an N-type region. Moreover, with respect to the siliconlayer 21, ion implantation of the P-type impurity is performed from thefront-surface side to form on the surface of the N-type region 27 apositive charge accumulated region 16 of a P-type (p⁺) (refer to FIG.9B).

Accordingly, the N-type region 17 of the photo sensor portion is formedof both the N-type region 25 which was formed from the rear-surface sideand the N-type region 27 which was formed from the front-surface side.

Subsequently, as shown in FIG. 9C, the wiring portion 3, in which aplurality of wiring layers 12 are formed, is formed on the silicon layer21 with the insulative layer 11 in between.

Furthermore, a protective coating is formed on the upper surface of thewiring portion 3, though not shown in the drawing. This protectivecoating is provided to prevent the wiring portion 3 from absorbingmoisture, so that the wiring layer 12 is not affected by the moisture.For example, a silicon nitride coating is formed as the protective filmby a plasma CVD method.

Next, as shown in FIG. 10A, an adhesive layer 34 is formed on a surfaceof a second supporting substrate 33, and the second supporting substrate33 is adhered to the wiring portion 3 with the adhesive layer 34 inbetween. Then, the heat treatment at a temperature of 400 degreecentigrade or less is performed for bonding. Since the heat treatment atthis time is performed after the wiring layer 12 was formed, thetreatment is performed at a lower temperature of 400 degree centigradeor less so that the wiring layer 12 can be prevented from the influence.A SOG (Spin On Glass) and a metal layer capable of metal-bonding can beused as the adhesive layer 34 in this case.

Next, as shown in FIG. 10B, a wafer is positioned again upside down.

Subsequently, the first supporting substrate 31 and the adhesive layer32 that are on the silicon layer 21 are removed using a back-grindmethod, a CMP (Chemical Mechanical Polishing) method, a wet-etchingmethod or the like, for example. As a result, the silicon layer 21 isexposed, as shown in FIG. 11A.

Next, as shown in FIG. 11B, an anti-reflective coating 28 is formed onthe silicon layer 21, and a color filter 5 and an on-chip lens 6 areformed thereon sequentially. A pad electrode for connecting to theoutside terminal or the like is also formed, though not shown in thedrawing.

As heretofore described, the solid-state imaging device of theback-illuminated type structure can be manufactured.

In addition, in a CMOS image sensor such as the solid-state imagingdevice 1 shown in FIG. 2, a peripheral circuit portion which performsdriving, control and others of the solid-state imaging device 1, areformed on the same semiconductor chip together with the solid-stateimaging device 1 which constitutes an imaging portion.

Therefore, though not shown in the drawing, a semiconductor region ofthe transistor and others of the peripheral circuit portion are alsoformed when the semiconductor region of the photo sensor portion isformed.

According to the above-described manufacturing method of thisembodiment, the solid-state imaging device having a similar structure tothe solid-state imaging device 1 shown in FIG. 2, namely the solid-stateimaging device in which the thickness of the silicon layer where thephoto sensor portion was formed, is 10 μm or less (preferably 5 μm orless), can be manufactured.

Therefore, in accordance with the manufacturing method of thisembodiment, a high sensitivity can be obtained in the visible lightrange, and mixed color and the shading caused by light incident on theadjacent pixels and electrically mixed color with the adjacent pixelscan be restrained; the dynamic range can be improved; and thesolid-state imaging device without the smear can be manufactured.

In this embodiment, since the silicon layer 21 is formed on the SOIsubstrate 24 in advance, an interface thereof is comparatively stableand the dark current generated on the interface can be reduced comparedwith the structure shown in FIG. 1.

Further, since the thickness of the silicon layer 21 can be controlledfavorably and the spectroscopic characteristic can be stabilized, theyield ratio of manufacturing can be improved.

Furthermore, with respect to the silicon layer 21, since the wiringportion 3 is formed on the front-surface side; the first supportingsubstrate 31 is bonded to the rear-surface side; and thereafter thefirst supporting substrate 31 is removed, the silicon layer 21 is notground, so that a mechanical damage to the silicon layer 21 can beprevented.

Further, in this embodiment, since the SOI substrate 24 is used, itbecomes possible to manufacture the solid-state imaging device withlower cost using an existing (commercially available) inexpensive SOIsubstrate 24, for example.

Particularly, in the manufacturing method of this embodiment, thecrystallinity on the interface of the active layer of the silicon layer21 can be improved by the heat treatment at a comparatively hightemperature when the first supporting substrate 31 is bonded, so thatthe solid-state imaging device with low noise can be manufactured.Further, since the p⁺ region 19 on the rear-surface side is formed byion implantation into the silicon layer 21 from the rear-surface side,the position of the p⁺ region 19 can be controlled easily in thevicinity of the interface of the rear-surface side of the silicon layer21.

Next, as another embodiment of the method for manufacturing thesolid-state imaging device of the present invention, another method formanufacturing a solid-state imaging device having the back-illuminatedtype structure similar to the solid-state imaging device 1 of FIG. 2 isexplained.

In this embodiment, the SOI substrate 24 shown in the vertical sectionof FIG. 7 is also used.

In the SOI substrate 24, the whole thickness thereof is 725 μm and thethickness of an intermediate layer (SiO₂ coating) 22 is 10 μm or lessfor example, and the thickness of the silicon layer 21 is 10 μm or less(preferably 5 μm or less).

First, as shown in FIG. 12A, the N-type region 17 constituting the photodiode, the p⁺ region 19 on the rear-surface side, the p⁺ region 16 onthe front-surface side and the N-type region which forms the floatingdiffusion 15 are respectively formed on the silicon layer 21 of the SOIsubstrate 24 by ion implantation. In addition, the matching mark 26 isformed for positioning the color filter and the on-chip lens. Note that,the N-type region 17 has different patterns in the upper portion and inthe lower portion, so that the ion implantation is performed twice, inwhich the lower portion is formed and then the upper portion is formed,for example.

At this time, when the thickness of the silicon layer 21 is 5 μm orless, the ion implantation can be performed using the photo resist (notshown in the drawing) as the mask; however, when the thickness of thesilicon layer 21 is more than 5 μm, the ion implantation is required tobe performed with a comparatively high energy using the hard mask suchas an oxide coating.

Next, the gate electrode 14 of the read-out transistor is formed on thesilicon layer 21 with a thin insulative coating in between.

Subsequently, as shown in FIG. 12B, the wiring portion 3 is formed onthe silicon layer 21, in which a plurality of wiring layers 12 areformed with the insulative layer 11 in between.

Further, a protective coating is formed on the upper surface of thewiring portion 3, though it is not shown in the drawing. This protectivecoating is to prevent the wiring layer 12 from being affected by thewiring portion 3 absorbing moisture. For example, the silicon nitridecoating is formed by a plasma CVD method.

Then, as shown in FIG. 12C, an adhesive layer 32 is formed on a surfaceof the first supporting substrate 31, and the first supporting substrate31 is adhered to the wiring portion 3 with this adhesive layer 32 inbetween. After that, the heat treatment at 400 degree centigrade or lessis performed to bond the first supporting substrate 31 to the wiringportion 3. Since the heat treatment at this time is performed after thewiring layer 12 was formed, temperature thereof is set to lower to be400 degree centigrade or less for preventing the wiring layer 12 frombeing affected. An SOG (Spin On Glass) and a metal layer capable ofmetal bonding can be used as the adhesive layer 32 in this case.

Then, as shown in FIG. 13A, a wafer is positioned upside down.

Subsequently, the rear-surface side is etched by a back-grinding method,a CMP (Chemical Mechanical Polish) method, a wet-etching method or thelike, for example, and the silicon substrate 23 and the intermediatelayer (SiO₂ coating) 22 of the SOI substrate 24 are removed.Accordingly, as shown in FIG. 13B, the silicon layer 21 is exposed.

Next, an oxide coating is formed by oxidizing the upper surface of thesilicon layer, though not shown in the drawing.

After that, as shown in FIG. 13C, an anti-reflective coating 28 isformed on the silicon layer 21, and a color filter 5 and an on-chip lens6 are formed thereon sequentially. A pad electrode for connecting to theoutside terminal is also formed, though not shown in the drawing.

Accordingly, the solid-state imaging device of the back-illuminated typestructure can be manufactured.

In addition, also in this case, the semiconductor region of thetransistor and others of the peripheral circuit portion is formed whenthe semiconductor region of the photo sensor portion is formed.Similarly, also the wiring of the peripheral circuit portion is formedas the wiring layer 12.

According to the above-described manufacturing method of thisembodiment, the solid-state imaging device having the same structure asthat of the solid-state imaging device 1 shown in FIG. 2, namely thesolid-state imaging device in which the thickness of the silicon layerwhere the photo sensor portion is formed, is 10 μm or less (preferably 5μm or less), can be manufactured.

Therefore, in accordance with the manufacturing method of thisembodiment, a high sensitivity can be obtained in the visible lightrange; mixed color and the shading caused by the light incident on theadjacent pixels and the electrically mixed color with the adjacentpixels can be controlled; the dynamic range can be improved; and then,the solid-state imaging device without the occurrence of smear can bemanufactured.

In the above embodiment, since the silicon layer 21 is formed on the SOIsubstrate 24 in advance, an interface of the silicon layer 21 iscomparatively stable, so that the dark current generated on theinterface can be reduced less than that of the structure shown in FIG.1.

Further, since the thickness of the silicon layer 21 can be controlledfavorably, the spectroscopic characteristic thereof can be stabilized,so that the yield ratio of manufacturing can be improved.

In addition, with respect to the silicon layer 21, since the wiringportion 3 is formed on the front-surface side and the silicon substrate23 and the intermediate layer 22 on the rear-side surface are removed,the silicon layer 21 is not ground, so that a mechanical damage can beprevented from affecting the silicon layer 21.

Furthermore, in the above embodiments, since the SOI substrate 24 isused, it becomes possible to manufacture a solid-state imaging devicewith lower cost using the existing (commercially available) inexpensiveSOI substrate 24, for example.

Particularly, in the manufacturing method of the above embodiment, sincethe heat treatment is performed at a comparatively low temperature of400 degree centigrade or less, the influence of the heat treatment onthe impurity regions of the source/drain and others of the transistorwhich is formed in a portion other than the photo resist of the siliconlayer is made small. As a result, since the channel length of thetransistor can be shortened more by applying the latest design rule, theminiaturization can be performed easily.

Note that, in the case where the thickness of the silicon layer 21 ofthe SOI substrate 24 has approximately 10% dispersion, the spectroscopiccharacteristic can not be influenced; however, dispersion with respectto the depth of the N-type region 17 and to the position of the P⁺region 19 on the rear-surface side may occur, even if the ionimplantation is performed under the same condition.

If the P⁺ region 19 on the rear-surface side is formed at the positiondeeper than the interface of the silicon layer 21 on the rear-surfaceside, the effectiveness that controls the dark current becomesinsufficient, and the noise occurs, which is undesirable. On the otherhand, if the P⁺ region 19 on the rear-surface side is formed at theposition shallower than the interface of the silicon layer 21 on therear-surface side, the P⁺ region 19 becomes an electrical barrier andthe amount of electric-charge which can be read out decreases, so thatthe sensitivity deteriorates.

Against the dispersion of the thickness of the silicon layer 21, suchmeasures are efficient, in which wafers with almost the same thickness(within the range in which the dispersion can be neglected) of thesilicon layer 21 are selected and used, for example; and in which thecondition (regarding the energy and others) of the ion implantation iscontrolled to be changed corresponding to the thickness of the siliconelayer 21, for example.

Further, in the above embodiment, as shown in FIG. 12A, the N-typeregion 17 and P⁺ region 19 on the rear-surface side are formed byperforming the ion implantation from the front-surface side of thesilicon layer 21; however, if the implanted ion can be activated at acomparatively low temperature, it is also possible that the portion onthe rear-surface of the N-type region 17 and P⁺ region 19 are formed byperforming the ion implantation from the rear-surface side, after thesilicon substrate 23 removed (after the process of FIG. 13B).Specifically, similarly to the previous embodiment of the presentinvention, the ion implantation can be performed twice from thefront-surface sided and from the rear-surface side.

Further, in each of the above-described embodiments, the N-type region17 is formed as a region in which the photo-electric conversion of thephoto sensor portion is performed, and P⁺ regions 16 and 19 are formedon the front-surface side and the rear-surface side of the N-type region17, respectively; however, the present invention can be applied to thestructure that has the inverse-conduction type respectively.

In addition, in each embodiment of the above-described manufacturingmethods, the SOI substrate 24 in which the SiO₂ coating is employed asthe intermediate layer 22 was used, and in the manufacturing methods ofthe present invention a layered substrate in which the siliconsubstrate, the intermediate layer and the silicon layer are laminated isused; however, it is also possible to use a layered substrate in whichother materials, for example, porous-silicon and other easily removablematerials are employed as the intermediate layer.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments and that various changes andmodifications could be effected therein by one skilled in the artwithout departing from the spirit or scope of the invention as definedin the appended claims.

1. A method for manufacturing a solid-state imaging device comprising atleast the steps of: forming photo sensor portions in a silicon layerover an intermediate layer over a substrate; bonding a first supportingsubstrate onto said silicon layer; and removing said substrate and saidintermediate layer.
 2. The method for manufacturing a solid-stateimaging device according to claim 1, wherein removing said substrate andsaid intermediate layer is done by using a back-grind, a CMP method or awet-etching method.